CVE-2026-33868
MediumMastodon has a GET-Based Open Redirect via '/web/%2F'
Published Mar 27, 2026 · Updated Sep 8, 2026 · Also known as BIT-mastodon-2026-33868, GHSA-xqw8-4j56-5hj6
Key Facts
Routine patching- CVE-2026-33868 is a medium-severity vulnerability (CVSS 6.1 v3.1) affecting Mastodon (Joinmastodon).
- It is not listed in any known-exploited vulnerabilities catalog. No public exploit code is known.
- EPSS estimates a 0.6% probability of exploitation in the next 30 days.
- Fixed versions are available, including Mastodon 4.3.21, Mastodon 4.4.15, Mastodon 4.5.8.
- Brandefense priority: routine patching, as no exploitation is known and its severity is moderate or low.
CVSS v3.1
6.1
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
EPSS (30-day exploit likelihood)
0.6%
Higher than 44.9% of all scored CVEs
Exploited in the wild
No known
Not in a known-exploited catalog
Public exploit code
None known
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Description
Mastodon is a free, open-source social network server based on ActivityPub. Prior to versions 4.5.8, 4.4.15, and 4.3.21, an unauthenticated Open Redirect vulnerability (CWE-601) exists in the /web/* route due to improper handling of URL-encoded path segments. An attacker can craft a specially encoded URL that causes the application to redirect users to an arbitrary external domain, enabling phishing attacks and potential OAuth credential theft. The issue occurs because URL-encoded slashes (%2F) bypass Rails path normalization and are interpreted as host-relative redirects. Versions 4.5.8, 4.4.15, and 4.3.21 patch the issue.
Affected Products
3 affected entries in total| Product | Affected versions | Fixed in |
|---|---|---|
| MastodonJoinmastodon | >= >= 4.5.0, < 4.5.8 | 4.3.21 |
Patched Releases
Mastodon
4.3.21, 4.4.15, 4.5.8
Timeline
Publicly disclosed
Mar 27, 2026
Last updated
Sep 8, 2026
Weakness Type
- CWE-601 URL Redirection to Untrusted Site ('Open Redirect')
References
FAQ
Common Questions.
Mastodon is a free, open-source social network server based on ActivityPub.
Affected: Mastodon (Joinmastodon) versions >= >= 4.5.0, < 4.5.8. The full list is in the CVE brief.
It is not listed in any known-exploited vulnerabilities catalog.
No, it is not currently listed in the CISA Known Exploited Vulnerabilities catalog.
No public exploit code is known.
Fixed versions are available, including Mastodon 4.3.21, Mastodon 4.4.15, Mastodon 4.5.8.
Brandefense priority: routine patching, as no exploitation is known and its severity is moderate or low.
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